Heretofore, there has been proposed a position sensor constructed in such a manner that a core is passed through a detection coil to detect change of impedance of the detection coil and to output a signal indicative of displacement of the core relative to the detection coil. FIG. 34 is a diagram schematically illustrating a detecting section of such a conventional position sensor. FIG. 35 is a graph showing a relation between displacement X of a core and alternate current impedance or AC impedance Zac of a detection coil 2 (sic). FIG. 36 is a diagram showing an entire configuration of the circuit of the position sensor. The AC impedance Zac has a component of a real number and a component of an imaginary number. In FIG. 35, the greater the displacement X is, the more the passing amount of the core 301 through the detection coil 302 is with the result that the AC impedance Zac increases. Alternatively, the position sensor may be configured in such a manner that the greater the displacement X is, the less the passing amount of the core 301 through the detection coil 302 is, so that the AC impedance Zac decreases as the displacement X increases.
In the above-constructed position sensor, generally, an alternate current is supplied to the detection coil 302 to detect an amplitude and a phase of a voltage detected at the opposite ends of the detection coil 302 so as to perform appropriate signal processing. The reason for supplying an alternate current is to obtain a voltage amplitude proportional to the AC impedance Zac of the detection coil 302.
In the case that the core 301 is made of a magnetic material, however, it is known that the temperature change ratio (temperature coefficient) of impedance Z of the detection coil 302 in passing the core 301 through the coil 302 is not uniform relative to the displacement X of the core 301 with the result that the temperature change ratio Δ(dZac/dt) increases as the passing amount of the core 301 through the coil 302 increases. As a result, it is necessary to compensate for an output voltage of the detection coil 302 in terms of circuit, configuration considering the temperature change, which makes the circuit configuration of the position sensor complicated.
U.S. Pat. No. 5,003,258, No. 4,864,232, No. 5,898,300, etc. propose a technique to solve the above drawbacks. FIG. 38 is a diagram disclosed in U.S. Pat. No. 5,003,258. What is inherently disclosed in these patent publications is an arrangement in which a detection coil 402 is so fabricated as to cancel a temperature change of impedance Z (inductance component) resulting from a magnetic member 421 of a core 401 and a temperature change of impedance Z (eddy current component) resulting from a nonmagnetic member 422 of the core 401.
Specifically, the above prior art proposes a technique of lessening dependency of the temperature coefficient of impedance Z of the detection coil 402 on the displacement X of the core 401 by providing the arrangement of the detection coil 402 and peripheral devices thereof in an attempt to solve the problem that the temperature coefficient of impedance Z depends on the displacement X. However, even in the above arrangement, there have occurred various problems such as increase of the number of parts constituting the position sensor, difficulty in positioning of parts relative to other parts, constraint in designing the detection coil, limited use of a sensor, and rising of cost in producing the sensor due to these reasons.
FIG. 39 is a graph showing a relationship between displacement X of the detection coil 302 and AC impedance Zac of the detection coil 302 shown in FIG. 34 in a state closer to an actual state than the one shown in FIG. 35. In FIG. 39, the AC impedance Zac shows a linear relation to the displacement X at an intermediate part of the stroke. However, the linearity is lost at the opposite ends of the stroke. Particularly, in the case where the passing amount of the core 301 through the detection coil 302 is small, the linearity is remarkably lost. This is considered because a lead end of the core 301 does not contribute to increase of impedance Z of the detection coil 302 as much as the remaining part of the core 301. Such a phenomenon is sometimes called as “end effect”.
Normally, the sensor is constructed in such a manner that linearity appears at an intermediate part of the stroke in correspondence to a desired displacement zone. However, desired linearity may not be obtainable for the aforementioned reasons, for example, in the case where the position sensor encounters dimensional constraint.
Next, described are some of the problems regarding the construction which the prior art has suffered from. One measure is proposed in the aspect of shape of the position sensor to improve the linearity of the position sensor. Specifically, there is a technique of increasing the ratio of the sectional area of the core 301 relative to the sectional area of the winding part on a bobbin 315 (see FIG. 34) by reducing the sectional area of the winding part on the bobbin 315 as much as possible. In such a case, it is preferable to set the clearance defined by the core 301 and the inner wall of the bobbin 315 (side surface opposing the through hole) corresponding to the winding part small.
As far as the bobbin 315 is made of a non-metallic material such as plastic, contact of the core 301 with the inner wall of the bobbin 315 does not greatly affect electrical characteristics (coil impedance or the like) of the position sensor. However, it is highly likely that contact of the core 301 with the inner wall of the bobbin 315 may obstruct smooth displacement of the core 301 relative to the detection coil 302, which may cause drawbacks such as deformation of the core 301 and generation of mechanical hysteresis.
In particular, it is highly likely that a rotary position sensor may encounter the aforementioned drawbacks because positioning of a curved core relative to a curved detection coil is difficult, and the core frequently contacts the inner wall of the bobbin.
Further, the rotary position sensor may encounter the following problems relating to coil winding. One of the problems is that uniform winding is difficult because the bobbin is curved. Thereby, a long time may be needed for winding a wire on the curved bobbin. Another problem occurs at the time of winding a wire on the curved bobbin. Specifically, the curvature of the bobbin after winding locally changes compared with the curvature thereof before winding due to a tension force exerted to the bobbin at the time of winding. Thus, smooth passing of the core through the bobbin is obstructed by the varied dimension of the inner wall of the bobbin corresponding to the winding part. In a worst case, displacement of a movable object is disabled on a half way of its displacement.